As the conductor density has increased with the evolution of the electronic circuit packaging arts, there has been a concomitant ever-increasing need to provide reliable electrical connections between multiple conducting lines on two circuit boards over long distances often in excess of 0.5 inches. A number of factors have been compounded to substantially increase the problems associated with providing these interconnections. The increasingly smaller dimensions of the high density interconnect pads (often having pitches and pad widths less than 0.020 inches and 0.010 inches, respectively), increasingly higher frequencies associated with the various circuitry to be interconnected (giving rise to signal and noise coupling problems with the interconnections and the associated difficulty in controlling impedances between the boards at the interconnections) are but a few of these factors. Moreover, due to the widely varying dimensions associated with contact pads and their spacing, it further became highly desirable to provide a universal connector readily adaptable to these variations to avoid the necessity to customize either the connector or the interconnect pad dimensions. Yet an additional problem in providing such interconnections also related to these increasingly smaller dimensions of the interconnect points. Due to the resultant susceptibility of these very small and some times delicate interconnect points to damage arising from the forces encountered in effecting interconnections, it became highly desirable to provide connector systems in which these forces were minimized, giving rise to zero insertion force connectors.
One approach in the art has been to provide for buckling beam type connections, the theory of such buckling being discussed in "Engineering Mechanics of Deformation Bodies", second edition 1969, Chapter 11, by Snyder Byars, which is incorporated by reference herein. Essentially the approach involved providing a very thin buckled wire member whose ends served as contact points to the two pads to be interconnected. By aligning the pads in vertical registry with the longitudinal axis of the beam and thence urging these contact pads toward respective ends of the beam until they matingly engaged therewith, the desired contact was made between the pads. The buckling properties of the beam were such that a relatively low and constant forces was exerted axially outwards at both ends of the beam toward the pads. Interconnections with good electrical integrity were thereby effected wherein these forces were relatively constant independent of the degree of axially inward compression exerted on the beam by the contact pads at either end.
Whereas this approach did provide for interconnection of relatively minute contact pads and further provided a substantial reduction in the forces which must be withstood by the pads and associated carriers during the act of effecting the connection, several serious problems nevertheless remained with this approach. First, the buckling beams typically were in the form of extremely thin whisker-like wires creating extremely difficult mechanical problems to overcome in fabricating a connector. The wires difficult to machine and manufacture, including problems controlling the dimensions thereof and other parameters including their spring properties, sharpness of the exposed beam ends for effecting good pad contact, characteristic impedance and the like. Numerous other problems were found to exist such as the need to package the beam so as to maintain proper orientation relative to the pads at all times and to do so with a minimum of complexity.
The problems associated with providing such a high density connector employing the buckling beam principle may be readily appreciated with reference to the article "High Density Double Contact Zero Insertion Force Connector", IBM Technical Disclosure Bulletin, Vol. 17, No. 2, July 1974 by Anzalone, et al, wherein there is disclosed an extremely mechanically complex connector employing a plurality of buckling beams. Not surprisingly, due to the aforementioned problems associated with buckling beam technology and the attendant costs, this approach more typically found application in single wire test probes or other test equipment wherein a small number of connections were required.
It is true that buckling beam technology was also employed in bed-of-nails testers and other equipment wherein numerous connections were desired. However such multiple buckling beam applications typically required interconnections to apparatus under test of only a temporary nature. Due to the high connector cost and complexity, they were not intended for the electronic packaging arts in general wherein numerous permanent interconnections were desired to be provided by a low cost connector for use on a mass scale (as in consumer electronics for example).
In an effort to provide the benefits of a buckling beam type connection while at the same time avoiding some of the hereinbefore noted drawbacks, yet another form of connector was developed for application with high density conductors known generally as elastomeric or polymetric connectors such as those manufactured by the PCK Elastometrics, Inc. Company, a unit of the Kollmorgen Corporation, 2940 Turnpike Drive, Hatboro, Penn. These connectors are typically made of a conductive silicone rubber or metal conductor supported by insulating silicone rubber bodies in a sandwiched fashion. Sections of the conductive material are alternated with sections of the insulating material sequentially so as to form a resilient strip. By sandwiching the elastomeric connector element under moderate pressure between two substrates, boards, or the like which carry conductive pads to be interconnected, interconnection is effected between preselected pads on the first and second substrates through the particular conductive sandwiched layers disposed between the pads. The interconnection thereby established is electrically isolated from adjacent such pad pairs by the sandwiched layers formed by the insulative material.
Numerous beneficial properties are provided by such elastomeric connectors over those offered by the buckling beam connectors of the prior art. They are of a relatively simple mechanical configuration resulting in ease of manufacture and suitability for high density applications wherein traces as fine as 0.001 inches with a pitch of 0.002 can be positioned on the elastomeric base. This provides opportunity for redundancy of interconnection between conductive pad pairs through a plurality of the conductive layers of the elastomeric strip. Moreover various other parameters associated with the connectors may be readily controlled to a relatively high degree of accuracy.
Nevertheless, notwithstanding the distinct improvement in many ways offered by this technology over the buckling beam approach, many extremely serious problems and limitations remained with these types of connectors.
One such deficiency with the elastomeric connectors is that the conductive surfaces thereof are typically relatively flat and blunt. The penetrating function associated with the pointed tips of the fine wires of the buckling beams is thus not available for effecting a more reliable gas tight seal with the contact pads. This sealing action is less susceptible to corrosion due to the wiping action of the buckling beam tips, and is not experienced in the elastomeric connectors. Moreover, with respect to the later it has been found that it is frequently more difficult in a given application to provide for a desired constant forcing function over time and variable distances over which the connector must be compressed to effect the interconnections. In other words, with respect to such polymetric connectors it has been found that their spring constant curve varies undesirably not only over time but as a function of the amount of deflection they must be subjected to during the connecting process.
Still further, no provision is made in either the buckling beam or elastomeric connector types of the prior art to facilitate multiple circuit layers in a direction normal to the longitudinal axis of the connector. For example, often times it is desirable to provide for a ground plane layer between the contact pads of the first and second boards to avoid aforementioned noise coupling problems between boards and the like. However the buckling beam and elastomeric connectors previously known did not readily provide facility for implementing ground planes for reducing this problem to manageable proportion.
These and other deficiencies of the connectors of the prior art adapted to high density conductors are overcome by the present invention. There is provided a connector which is of a simple mechanical construction that is easy and inexpensive to manufacture and highly versatile in providing opportunity to finely control numerous parameters associated with the conductive strip (including the geometric properties of each beam to a high degree of precision such as lengths, widths, tip sharpness and spacing). Moreover additional flexibility is provided by multiple conductive layers in a direction normal to the longitudinal axis of the conductor whereby, for example the aforementioned ground plane may be included to effect improved noise performance of the connector. These and other benefits of the present invention are provided.